LB9GTA

iTrader: (1)

Here's my combo
91 gta - stock shortblock 400sbc with afr 195 heads and LT1 intake(hp peak at 5500rpm)
th350 with 3500stall foot braking
3620lbs with me in the car
28" MT drag radial
ford 9" with 3.75 gear.
125 shot of dry nitrous ( with potential to go to 175hp with good race gas)

I am crossing the stripe at 6000rpm (by the end of the 1/4, it runs out of steam)
Best pass to date is 10.97 @ 120
6.90 @ 97 at the 1/8th
1.4 60ft.

Should I go to a 3.50 rear gear?
Do I have too good of a 60ft and sacrificing the top end speed for better ET?

Dialed_In

iTrader: (3)

What heads/cam/intake are on the car? Have you dyno'd it to know where the power drops off?

LB9GTA

iTrader: (1)

Heads are AFR 195 eliminator
Cam is XFi 280 (.550 lift and ext) (236/241 dur)
Intake is a LT1 converted intake (like a mini ram)
My compression is 9.0-1
Headers are 1 7/8 Ed Quay long tube thru single 4"
Best et on the motor is 12.2@ 108

I did not have this combo on the dyno but I had my last combo on the dyno.
383 with 11-1 compression with 1 5/8 shorty open headers. Everything else was the same
This combo made 377 rwhp at 6200rpm and best of 12.09 @ 110.5

So I would guess the 400sbc combo would make about the same, but at slightly less rpm.

Judging by go-pro vids of my car, you can see right before the stripe that the power runs out

Dialed_In

iTrader: (3)

Just find out where the power goes away and gear it to trap that RPM. Tough to compare the two combos considering the lower CR and the giant tubes you've got on it now.

FRMULA88

I would leave the gearing alone and look at raising the power band of the engine.

Find the "bottle neck" and address that instead because a 40o sbc can easily rev to 6000-6500. 5500 is too low


Going to a higher gear will just make the car launch slower, with a 1.4 60 foot the car should run mid low 10s at over 125 MPH provided it can rev bit more.

with those tall tires 3.75 is spot on for your 1/4 mile combo. This is obvious because of your 60' time & you are crossing the line at 6000 rpm... The real issue is your motor is running out of steam at 5500. A higher gear will not solve that.

A cam shaft / valvetrain / intake change would produce better results..

what ratio rocker arms are you using? you could try by increasing valve lift first to pick up some RPM. (HP)

LB9GTA

iTrader: (1)

I not sure if I mentioned this before but I want to go faster and I have 50 more hp of juice to spray.

The bottleneck for my engine is the stock rods and pistons.

I would think that a gear install would be much easier/cheaper than a motor rebuild.

I also found this site and I have way too good of a 60ft for a 10.97
http://www.wallaceracing.com/etcalc.php

60 Foot 1.52
330 Foot 4.40
660 Foot 6.92
660 MPH 98.20
1000 Foot 9.11
1/4 Mile ET 10.97
1/4 Mile MPH 120.78

60 Foot 1.4
330 Foot 4.04
660 Foot 6.36
660 MPH 106.95
1000 Foot 8.36
1/4 Mile ET 10.07
1/4 Mile MPH 131.55

AlkyIROC

Your cam/intake/heads has peak HP at 5500 rpm and you're running out of steam as you cross the finish line at 6000 rpm. Changing gearing isn't going to help you until you increase the rpm range of the engine.

Not sure what camshaft is in the engine but the converter stall speed might not be matched. Too low or too low a stall speed can affect performance. Changing gears normally means a different stall speed as well.

I run 4.86 gears in my race car but also have a 6200 stall converter shifting around 7400 rpm. Dyno software recommends gears closer to 4.57. I tried a different gear set and the car slowed down. The only way I would have been able to pick up the lost ET was with a tighter converter. I went back to the 4.86 gears and the car loves them.

LB9GTA

iTrader: (1)

Well my thinking is that if I put in a 3.50 gear, I can keep the engine in the sweet spot longer rather than run past the HP peak.

I eventually want to run a 10.50 index class this coming up summer.
With my gears, I can't run anymore nitrous because I'm on the limiter with running 120mph.

AlkyIROC

Well with so many different combinations and how they react, the only way you're going to know for sure exactly what a gear change will do is to install a different ratio and make some passes. I still think you'll need to do a stall change along with a gear swap.

LB9GTA

iTrader: (1)

Stall change by looser/tighter/higher/lower?

83 Crossfire TA

iTrader: (2)

The XFi 280 has is a 230/236 with .576"/.570" lift, do you have something different? Are you running 1.5 rockers (Comp specs their xfi cams with 1.6 rockers)? I'm actually a bit surprised that that those heads, cam and intake are falling flat before 6000rpm, I've had a stock LT1 that was quite happy to rev to 6400 (after moving the pcm rev limiter) and actually ran fastest just tagging the limiter on each shift.

mw66nova

iTrader: (13)

do you think it has something to do with the larger cubic inches though? cause the lt1 having 50 less cubes means that much less volume at the same RPM.

LB9GTA

iTrader: (1)

No, mine is this
http://www.compcams.com/Company/CC/c...?csid=122&sb=2
236/241
.552 int, .547exh using 1.6 rockers
Hyd flat tappet, my block is from 1974! I'm using 1.6 rockers and I still have the stock 5.565 rod. That's why it runs outa power.
With this same cam in my last engine (383 stroker with same heads and intake) topped out at 6200rpm hp peak.

FRMULA88

Adjust your rev limiter, it should be at least 1000 RPM higher than "peak" HP RPM.

MSD boxes start to cut out 500 RPM before the actual limit.

A well built 400 SBC should rev to 6000-6500 no problem and make power up there. Even if peak power occurs at 5500 you may have long shift it to keep the power band in the sweet spot of the power band. Going to a higher gear will make the rpm drop between shifts greater, so this is counter productive. This only make sense if you were making ALOT more HP. turbo and pro-charger cars with ALOT of boost need less gearing.

Try playing with the rev limiter first because 6000 rpm thru the traps and you say your are on the limiter means, to me, you have the right RPM for the gearing but too low a rev limit setting.

How are you shifting the car? are you manually shifting with a shift light or leaving it in "D" ? if you are auto shifting you can fine tune the shift point by adjusting the governor weights and springs...

FRMULA88

FWIW my car with 26" tall tires and 3.70 gears and no overdrive has a top speed of 150 MPH.

you car with slightly taller tires and similar gearing would have a little more top speed.

but top speed is not 1/4 MPH or ET

you short time is great... why ruin that 1.4 60' ? All you need to do is fine tune the car on the big end..

brandoz28

iTrader: (1)

I would try the 3.50 gear. If it slows down on the 60ft, spray it harder or loosen up the converter.

mw66nova

iTrader: (13)

if rpm isn't good for stock rods, i'm not certain how more spray IS, lol!

AlkyIROC

My old short rod 383 had a solid flat tappet cam. Can't remember the duration but it was around .550 lift. Too many runs shifting at 7000 rpm finally failed a stock rod bolt and took out the engine.

My combination of parts was all wrong. 3.27 gears with 26" tall tire. TH350 with a 3800 stall converter. 2.02/1.60 ported out camel heads. Single plane intake with an 850 DP carb. Cast pistons, stock 400 rods and rod bolts.

Engine made lots of power and could easily hit 7000 rpm but with stock internals, it only lasted a couple of years (200 1/4 mile passes) doing that. It ran well enough to get me very high 11 passes but at 117 MPH which is much faster than a 12 second pass should be. Your 12.2 at 108 is just about right.

I agree with the above post that the rev limiter needs to be increased. I just looked at your cam card. That camshaft only makes power to 6400 rpm Setting the rev limiter around 6800 should be fine as you really shouldn't be going much higher than 6400. With the bottom end of the powerband at 2400, your 3500 stall is just fine for strip only use but if you're running out of steam at 6000 when you cross the stripe I only have a couple of suggestions.

Might be hitting the rev limiter. Bump it up.

Engine isn't breathing enough and can't get enough air into the cylinders at that rpm. Bigger valves and or better flowing heads.

Valves are floating at high rpm. Valve float is when the spring is slamming the valve back down onto the seat as the lifter is falling off the backside of the cam lobe. If the spring seat pressure isn't high enough, the valve will bounce as it hits the seat which will decrease cylinder pressure and cause a loss of power. The higher the rpm, the faster the valves are opening and closing and the more noticeable valve float is. The valve springs recommended for that camshaft should have at least 125 pounds of seat pressure.

Your camshaft also needs gear due to the higher powerband range. Your 3.75 are much better suited than something in the mid 3's. If you can get the engine up to it's peak rpm range, 4.10 gears would be better as the extra rpm would let you stay in each gear a little longer.

If everything is set up properly, you should be crossing the finish line at or just after your peak shifting rpm. Buzzing the engine slightly higher than your shift point just as you cross the finish line is perfect.

FRMULA88

exactly what Alky & I suggest

Adjust your rev limiter.


FWIW you can install better valve springs and rocker arms

for a fraction of the price of that rear gear swap.

keep up posted and good luck !

AutoRoc

iTrader: (1)

I'd port and Mill the heads for more squeeze... do a 28" bias ply tire. It'll stretch to 29+" on the big end.

You have a badass stump puller otherwise. Runs good though! I wish my car cut 1.4 60fts!

brandoz28

iTrader: (1)

It's more of an issue with the rods being short holding it back. those stock 400 rods will hold more than most people will give them credit. If he's only at a 125 shot now, and looking to go to 175, I would try that first.

On the other hand, it's all going to depend on if he wants it to run it's best on motor and give up a little on the juice or the other way around.

LB9GTA

iTrader: (1)

I have a manual shift th350 and I shift it at 5500.

After looking into my valve springs I may have found my top end problem.
I have this spring (btw, I bought these in 2005)
http://www.summitracing.com/parts/cca-981-16
I read that thay may have valve float at 6400-6800rpm.

Should I get a spring with less coil bind height and more spring rate?
Like this one?
http://www.summitracing.com/parts/cr...4-16/overview/

brandoz28, I would rather slow my n/a passes down and get more out of my nitrous passes. I want to be able to run closer to 10.5's this year.

AlkyIROC

Your 981 springs have 105 pounds of seat pressure if installed at 1.700". They'll coil bind at 1.150" which means anything over 0.550" lift is coil binding and there should be 0.030" to 0.060" left over in the springs from max lift to coil bind. Those springs are stock replacement for an OEM camshaft. Even the cam listing says not to use stock springs.

Right there I'd say you have the wrong springs installed for that camshaft. If they're installed at 1.800" then the seat pressure is something like 60 pounds (mega valve float)

I have no specs on those Crower springs for a comparison.

Like I already said. You need springs with at least 125 pounds on the seat. The recommended springs for that cam are Comp Cams 26918-16. They're a beehive spring. At a recommended 1.800" install height, they have 130 pounds on the seat. I wouldn't install them at 1.700" height with your camshaft as it would be too close to coil binding.

Buy the proper valve springs and have the install height set to 1.800" before going back to the track.

LB9GTA

iTrader: (1)

Wow!
Thanks for the info!
I can't believe I have ran that long on the wrong springs!

I gotta do some research and see what I have for retainers and valve locks and springs and make sure they're right.
And also, what shim I have in there and if its the right height.

AlkyIROC

You need a tool to measure the install height. You should always have a shim or locater under the spring with aluminum heads. Iron heads can get away without a shim.

Measure the install height with the retainer and keepers on the valve stem. If it's greater than 1.800 then put a shim under the spring to get it as close as possible. Shims normally come in .015, .030 and .060" sizes.

If the measured height is less than 1.800, you'll need different keepers to gain a little more height. Worst case is that the spring pockets will need to be machined to allow more install height. Normally the valve guide will also need to be machined down so that the bottom of the retainer doesn't hit the top of the guide at full lift.

1.800 install height shouldn't be too much of an issue but if you have a cam with a really high lift then an even higher install height is required with bigger springs. That's when you need to look at longer valves which will change your valve train geometry which means you needs different length pushrods.

So you want to run a really high lift cam? Welcome to proper engine building.

FRMULA88

[QUOTE=LB9GTA;5674254]I have a manual shift th350 and I shift it at 5500.

QUOTE]

Get the valve train sorted out before trying another suggestion.

IMHO your engine has lived this long because or your conservative shifting but this is hurting your ETs.


5500 is way too low of shift point. (super short shifting) Ideally you want to shift at the peak of the power band to keep the revs in the power band so the engine is making the most power and not trying to get back into the sweet spot.

In your case you want to try 1-2 & 2-3 shift at 6400.

Right now shifting at 5500:
RPM after 1-2 shift is 3,317
RPM after 2-3 shift is 3,618


BUT shifting at 6400:
RPM after 1-2 shift is 3,860
RPM after 2-3 shift is 4,211

Just adjusting your shift point you will pick up some ET and MPH

this is something you will have to play with... the engine may like 6,400 or 6,200 or 6,600 but shifting at 5500 that is pretty much the stock shift point.


You need to make sure the valve train is up to the task.

just raising your shift point will take advantage of your 3.75 rear gear.

LB9GTA

iTrader: (1)

I called my buddy that helped me put the engine together back in 2005 and he gave me some insight that I forgot about.

I bought the AFR 195 assembled. So they had the 8017 valve springs on them
http://www.airflowresearch.com/valve_spec.php
135lbs on the seat, 1.05 coil bind height, 360lbs spring rate

So I bought the stock oem comp cam springs (981-16)(but took out the dampner spring) so that I could break in the 280xfi cam. I took out the springs (gave them to my buddy for helping, I'm a nice guy) and then put the 8017's back on the afr head.

So I do have decent springs on the engine now, but they're old.
So should I buy new springs?
Should I buy the 8019's from AFR?
155lbs on the seat, 1.08 coil bind height, 428lbs spring rate. standard spring

Or buy the 26918 from comp? (behive spring)

I read that the 26918's are lighter and allow more rpm.

Thoughts?

mw66nova

iTrader: (13)

i like the behives. also not sure you'd need 428lbs open seat pressure.

AlkyIROC

The 8017 springs should be fine providing they're not worn out. You need a tool to check how much pressure is required to open the valve. If the spring pressure is low then new springs should be installed. The springs can also be tested on a bench mounted spring checking tool but it's unlikely anyone but an engine machine shop will own one.

That's also assuming the springs were installed at the correct recommended install height. If they are rated that much pressure at 1.700" and were installed at 1.800" then the seat pressure may be too low. Going to a higher spring rate at the higher install height can bring the required seat pressure back up to where the camshaft wants it.

lighter springs and higher rpm don't mix. I spin my engine into the mid 7000 range but have huge valves. I run triple springs that have around 300 pounds seat pressure. My valves don't float.

LB9GTA

iTrader: (1)

Here's a link to a vid of my car shifting at 5500rpm.
It does seem to slump from the 2-3 shift.
I had a 125hp shot on this run and I shut the juice off at 6000rpm.
Notice that right at the stripe the nitrous shuts off.

LB9GTA

iTrader: (1)

Another update!
I called AFR's tech line. I told them that I bought the heads in 2005 and he said they came with the 8020 valve springs!
1.437 OD
120# @1.800 on seat
300# @ 1.250 open
Coil bind height 1.090
spring rate - 325
gross max lift .550
max rpm - 6100-6300 RPM
He said that spring is totally wrong for my cam (coil binding and valve float)
He advised me to use the 8017 springs.
So I think I am going to return the gears and get the proper springs and let it eat.

Thanks for the info guys!

mw66nova

iTrader: (13)

just be glad you haven't had a failure yet man...

LB9GTA

iTrader: (1)

Update.
I took the valve covers off and noticed that I already have the 8017 springs.
Got them checked for spring pressure. They were only down 5lbs, so well within tolerance.
So now I know that the springs aren't floating or binding.

I might check the cam for lobe wear.